How many concrete blocks do I need per square foot?

Standard 8×8×16 CMU blocks: 1.125 blocks per square foot of wall face. This accounts for the 3/8-inch mortar joint that brings each block's coverage to exactly 8×16 inches nominal. For a 100 sq ft wall: 100 × 1.125 = 112.5 blocks → order 122 with 8% waste.

How do I calculate the number of blocks for a wall?

1. Calculate net wall area (length × height, minus openings). 2. Multiply by 1.125. 3. Add 8% waste. Example: 20 ft × 6 ft wall with one 3×7 door: Net area = (20 × 6) − (3 × 7) = 120 − 21 = 99 sq ft. Blocks = 99 × 1.125 = 111.4 → with 8% = 121 blocks ordered.

Do I subtract window and door openings when calculating CMU blocks?

Yes — subtract the area of all openings before multiplying by the block factor. Window area = width × height. Door area = width × height. Then multiply the remaining net area by 1.125 blocks/sq ft. Add 8% waste to the result.

How much mortar do I need for a CMU block wall?

Approximately 0.26 cubic feet of mortar per square foot of standard 8-inch CMU wall. An 80-lb bag of mortar mix yields approximately 0.66 cubic feet. For a 100 sq ft wall: 100 × 0.26 = 26 cubic feet ÷ 0.66 cu ft/bag ≈ 40 bags of mortar mix.

How to Calculate Concrete Blocks for a Wall: CMU Block Count Formula

Calculate the number of CMU concrete blocks needed for a wall. Covers block coverage per square foot, mortar joints, waste factor, and how to account for openings.

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Standard CMU Block Dimensions

The most common concrete masonry unit (CMU) is the 8×8×16 block: - Nominal size: 8 inches tall × 16 inches long - Actual size: 7⅝ inches tall × 15⅝ inches long (3/8-inch mortar joint makes up the difference)

With mortar, each course = 8 inches high; each block = 16 inches long.

Blocks Per Square Foot of Wall

Each standard 8×16 block covers: 8 in × 16 in = 128 in² = 0.889 sq ft

Blocks per square foot = 1 ÷ 0.889 = 1.125 blocks/sq ft

This is the standard factor used in the industry: 1.125 blocks per square foot of gross wall area.

Step-by-Step Calculation

Step 1: Calculate gross wall area (including openings) Area = Wall Length × Wall Height

Step 2: Subtract opening areas Net Area = Gross Area − Door Areas − Window Areas

Step 3: Calculate block count Blocks = Net Area × 1.125

Step 4: Add 5–10% waste Total Order = Blocks × 1.08 (8% waste typical)

Worked Example

Wall specs: 30 ft long × 8 ft tall, two windows (3 ft × 4 ft each), one door (3 ft × 7 ft)

Gross area: 30 × 8 = 240 sq ft
Window area: 2 × (3 × 4) = 24 sq ft
Door area: 3 × 7 = 21 sq ft
Net area: 240 − 24 − 21 = 195 sq ft
Blocks: 195 × 1.125 = 219.4 blocks
With 8% waste: 219.4 × 1.08 = 237 blocks → order 240 blocks

Accounting for Mortar

Mortar is applied in bed joints (horizontal) and head joints (vertical). Standard 3/8-inch mortar joints are already accounted for in the 1.125 factor (based on nominal 8×16 dimensions).

For mortar volume, approximate: 0.26 cubic feet of mortar per square foot of 8-inch wall (for standard CMU).

For a 195 sq ft wall: 195 × 0.26 = 50.7 cubic feet of mortar = approximately 47 80-lb bags of mortar mix (each makes ~0.66 cu ft after water addition).

Block Types and Weights

Block Type	Nominal Size	Weight
Standard (8×8×16)	8 in × 16 in face	35–42 lbs
Lightweight (8×8×16)	Same	22–28 lbs
Half block (8×8×8)	—	17–21 lbs
Lintel block	8 in × 16 in	32–38 lbs
Corner block	8 in × 16 in	35–42 lbs

Special Considerations

Corners: Corner blocks are needed at every course for exterior corners — 2 per course per corner.

Lintels: Openings need lintel blocks filled with rebar and grout to span doorways and windows. Calculate the lintel block count separately: opening width ÷ 16 in = number of lintel blocks per course (typically 2 courses per opening).

Bond beam: Many walls include a bond beam course at the top with continuous rebar — these use special bond beam blocks. Count 1 block per 16 inches of wall length.

Related Guides

Frequently Asked Questions

How many concrete blocks do I need per square foot?: Standard 8×8×16 CMU blocks: 1.125 blocks per square foot of wall face. This accounts for the 3/8-inch mortar joint that brings each block's coverage to exactly 8×16 inches nominal. For a 100 sq ft wall: 100 × 1.125 = 112.5 blocks → order 122 with 8% waste.
How do I calculate the number of blocks for a wall?: 1. Calculate net wall area (length × height, minus openings). 2. Multiply by 1.125. 3. Add 8% waste. Example: 20 ft × 6 ft wall with one 3×7 door: Net area = (20 × 6) − (3 × 7) = 120 − 21 = 99 sq ft. Blocks = 99 × 1.125 = 111.4 → with 8% = 121 blocks ordered.
Do I subtract window and door openings when calculating CMU blocks?: Yes — subtract the area of all openings before multiplying by the block factor. Window area = width × height. Door area = width × height. Then multiply the remaining net area by 1.125 blocks/sq ft. Add 8% waste to the result.
How much mortar do I need for a CMU block wall?: Approximately 0.26 cubic feet of mortar per square foot of standard 8-inch CMU wall. An 80-lb bag of mortar mix yields approximately 0.66 cubic feet. For a 100 sq ft wall: 100 × 0.26 = 26 cubic feet ÷ 0.66 cu ft/bag ≈ 40 bags of mortar mix.

Last updated 7/28/2026